I suppose if every player wasn't allied, nobody would hear their message. Your trigger could then pass that message to the specified player using substrings. You could make other triggers to simulate an alliance with other players. "Allies" could be neutral with shared vision to prevent auto attacking. Experience and what not could to triggered to be shared, etc, etc.
Suppose it could work. Maybe there's another solution? I know players can already privately message if they're playing online but LAN and single player won't have this functionality.